1. Field of the Invention
This invention relates to bonding permanent magnets in steel construction and more particularly to interlocking bonding geometries for bonding permanent magnets to steel portions of magnet assemblies. This bonding construction is particularly well suited for use in bonding permanent magnets to steel motor housings.
2. Description of the Related Art
Permanent magnets are pieces of material that retain a magnetic field after being exposed to a magnetizing field. Once magnetized they do not need energy input to maintain their field. As a result, they have found numerous uses. Of particular interest is their use in electric motors. Electric motors generate torque by the interaction of magnetic fields. Prior to the development of strong permanent magnets, electric motors employed both stationary and moving electromagnets to generate the interacting magnetic fields needed to generate torque.
Electricity is required to produce magnetic fields within electromagnets. The result is often a substantial amount of electrical loss within the electromagnets themselves. In addition, since power has to be supplied to moving electromagnet windings, some sort of brush system is also required. General electric developed the first permanent magnet material that was strong enough to replace some of the electromagnets in electric motors. This magnet material was called Alnico and shortly thereafter several grades became commercially available. Improved permanent magnets made possible permanent magnet motors.
A permanent magnet motor is an electric motor employing permanent magnets as the fixed magnetic field and electromagnets as the changing interacting magnetic field. The use of permanent magnets in electric motors improved their reliability and reduced manufacturing costs. There are numerous configurations possible with permanent magnet motors. One common configuration involves placing curved permanent magnets against a motor housing and having a rotatable electromagnet assembly located centrally within the field of the permanent magnets. The bonding of permanent magnets into motor housings may be accomplished using a balloon to press the permanent magnets against the housing and using epoxy resin to adhere the magnets firmly into place. When the epoxy finishes curing, the balloon may be deflated and removed. Another method that can be used for bonding permanent magnets in magnetic assemblies is disclosed in U.S. Pat. No. 4,011,120 titled “Method For Fastening Ceramic Magnets To A Flywheel Using Centrifugal Force”. U.S. Pat. No. 4,011,120 discloses the placement of permanent magnets having applied adhesive against the inner surface of a flywheel and spinning the flywheel in a special fixture having positioning pins. The centrifugal force created by the spinning action holds the magnets in place until the adhesive sets.
The torque generated in permanent magnet motors depends on the field strength of the permanent magnets and the field strength of the interacting field of the electromagnets. The stronger the field of the permanent magnets the greater will be the torque. The stronger the field of the electromagnets, the stronger will be the torque.
Permanent magnets can withstand a limited demagnetizing field. If too much current is applied to the electromagnets of a permanent magnet motor the permanent magnets will demagnetize, and the motor will cease to function properly. Because of this, the power supplied to permanent magnet motors needs to be limited.
As time progressed so did permanent magnet development. Today, there are strong permanent magnet materials that resist the forces of demagnetization and have inherently strong magnetic fields. Permanent magnets made from certain rare earth compositions such as neodymium iron boron can have very good properties for use in permanent magnet motors.
A bit needs to be said about the standard design of permanent magnet motors with respect to the permanent magnets themselves. In the standard configuration, curved permanent magnets are placed against a steel motor housing. The magnets face each other with opposite poles aligned. Magnetic flux travels through the steel motor housing from the back of one magnet to the other. The central portion where the rotating electromagnet assembly spins is open with the field of the permanent magnets traveling through. The pathway of magnetic flux forms closed loops between the permanent magnets, the steel motor housing, and the air space in the central portion of the motor. This pathway is sometimes referred to as the “magnetic circuit”.
When magnetic flux travels through air, it is said to be traveling through an “air gap”. Generally speaking, the greater the air gap, the less will be the torque of the motor. Because of this, designers of electric motors often reduce this air gap to the bare minimum. It should be noted that the thin layer of epoxy bonding the permanent magnets to the housing can be considered to be an air gap due to the fact that like air, epoxy is a poor conductor of magnetic flux. Examples of this are numerous and thin non-magnetic materials are often used as part of magnet bonding and/or holding assemblies. An example of this can be found in U.S. Pat. No. 4,920,634 titled “Permanent Magnet Rotor With Magnet Retention Band” incorporated herein by reference. U.S. Pat. No. 4,920,630 discloses the use of a non-magnetic retention band wrapped around the rotor magnets of a permanent magnet electric motor. The opposite ends of the band are extended inwardly in the space between the magnets and firmly secured to the rotor core. In circumferentially spaced relation to the secured ends, a further securement of the band is established between the band and the rotor core. The apparatus and method disclosed in U.S. Pat. No. 4,920,634 has the advantage of allowing the band to be of minimal thickness thereby minimizing the air gap.
Strong rare earth permanent magnet motors can be made small in size, light in weight and very powerful. Because of this, it is often the case that permanent magnet motors employing rare earth magnets run hot. The result is that the epoxy holding the permanent magnets to the motor housing may be compromised resulting in bond failure. Once this happens, the permanent magnets slide around loosely within the motor and may jam the rotor.
In addition to thermal instabilities to the bond between permanent magnets and their motor housings, high torque values are often associated with rare earth permanent magnet motors. These high torque values add further strain to the bond between the motor housing and permanent magnets.
It is an object of this invention to provide good bonding of permanent magnets to steel in magnetic assemblies.
It is an object of this invention to provide good bonding of permanent magnets to motor housings.
It is a further object of this invention to provide bonds that are stable to high temperatures.
It is a further object of this invention to provide a strong bond between a permanent magnet and motor housing capable of withstanding large torque values.
Finally, it is an object of this invention to provide a bonding method between a permanent magnet and motor housing that minimizes magnetic path air gaps.